A liquid crystal display element is composed of two substrates which are constantly spaced and liquid crystal which is injected into a gap between the two substrates. It is desirable that the gap between the two substrates is uniform so as to obtain a liquid crystal display element which provides satisfactory display. Moreover, it is desirable that a liquid crystal display element has no irregularity of a gap. The irregularity of a gap is caused by a partial distortion of the substrate due to pressing and by a distortion of the substrate due to a swelling of liquid crystal at high temperature.
Therefore, in order to obtain an uniform gap, generally, spherical gap holding materials are scattered between the substrates.
As a method of improving an uniformity of a gap in a liquid crystal display element, a technique that a diameter of a spacer which is a gap holding material in a liquid crystal panel is allowed to be smaller than a thickness of sealant around the spacer, and that an inside of the liquid crystal display element is allowed to be in a negative pressure state is disclosed in Japanese Examined Patent Publication No. 59-18685/1984 (Tokukosho 59-18685). However, when this method for allowing the inside of the liquid crystal display element to be in the negative pressure state is used in the case where the element is left in low temperature, air bubbles which are substantially vacuum are liable to occur. This is because since the inside of the element is under negative pressure, a change in capacity of the substrates and a change in in side capacity of the liquid crystal display element do not follow the decrease in volume of liquid crystal due to low temperature.
In order to prevent the air bubbles from occurring at low temperature, Japanese Unexamined Patent Publication No. 1-96626/1989 (Tokukaihei 1-96626) discloses a method of using a mixture of rigid granules and polymeric particles including epoxy radical.
In addition, as a method of preventing irregularity of a gap at high temperature and irregularity of a gap due to pressing, Japanese Unexamined Patent Publication No. 63-6527/1988 discloses a method of using glass fiber and plastic beads which are larger than the glass fiber. Moreover, as another method, Japanese Unexamined Patent Publication No. 62-150224/1987 (Tokukaisho 62-150224) discloses a method of using a mixture of a hard gap holding material which does not show heat fusibility and a soft gap holding material which has an average particle diameter of not more than twice as a particle diameter of the hard gap holding material and which shows heat fusibility.
When the above-mentioned conventional gap holding material with a wide distribution of a particle diameter is used, an amount of the holding material which contacts with upper and lower substrates contributing to gap holding in a liquid crystal display element is small, so the substrates can bend freely to a certain extent. Therefore, when a liquid crystal display element is transported and handled, and when it is handled after being installed to a product such as a personal computer, if excessive impact is given to the liquid crystal display element, the impact can be absorbed by bending of the substrates.
As a result, the impact seldom causes irregularity of a gap, and quality of a liquid crystal display element is seldom lowered. Furthermore, since distribution of a particle diameter is wide, irregularity of a gap occurs in a liquid crystal display element at the beginning. Therefore, even if irregularity of a gap occurs due to the impact, it is not noticeable.
However, in the case where the mixture of glass fiber and plastic beads, the mixture of a hard gap holding material and a soft gap holding material which shows heat fusibility and the mixture of rigid granules and adhesive polymeric particles including epoxy radical material with low hardness are used, the following points are not specified. Namely, a difference in a particle diameter between a gap holding material with low hardness and a gap holding material with high hardness and a mixing ratio of them, hardness of a gap holding material with low hardness and particle diameter accuracy of the gap holding materials are not specified.
Furthermore, optimum combination of the gap with high hardness and the gap holding material with low hardness is not clear. These points are important factors which greatly affect improvement in gap uniformity and a gap holding characteristic at the time of high temperature and impact. For example, if the hardness of a gap holding material with low hardness is too low, gap holding at the time of high temperature and of impact is not effective.
Incidentally, recently, enlargement of an area of a liquid crystal panel using a liquid crystal display element, thinning of substrate and high contrast of a panel characteristic further require gap accuracy of a liquid crystal display element. In prior arts, a resin-type gap holding material in which CV value representing particle diameter accuracy is about 6% was generally used. Therefore, it is hard to satisfy the demand of the gap accuracy.
However, as a spherical gap holding material composed of resin, recently, one with high particle diameter accuracy which is disclosed in PCT Unexamined Patent Publication No. 6-503180/1994 (Tokuhyohei 6-503180) can be obtained, for example. The above Publication discloses the case where a liquid crystal display element is produced by independently using the gap holding material with high particle diameter accuracy. Since this gap holding material is composed of resin, when a liquid crystal display element is cooled to a low temperature, the liquid crystal display element is bent to extent that can correspond to decrease in volume of liquid crystal, and its capacity can be also decreased. Therefore, vacuum air bubbles which are seen when the gap holding material composed of an inorganic material is used do not occur, so it is possible to improve gap uniformity.
When a resin gap holding material with high accuracy of a particle diameter in which 10% compression modulus is 214 to 600 kg/mm.sup.2 and particle diameter accuracy (CV value) is not more than 4% is used, gap accuracy can be improved. Nevertheless, since the above gap holding material has high particle diameter accuracy, most gap holding material which spreads between a pair of substrates contributes to gap holding between the upper and the lower substrates, so the pair of substrates is held rigidly. A degree of freedom from bending of the substrates becomes smaller compared to a liquid crystal display element using a conventional resin gap holding material with slightly wide distribution of a particle diameter.
If a degree of freedom from bending of the substrates becomes small, image quality of a liquid crystal display element is remarkably deteriorated. The reason for this will be mentioned in the case where the liquid crystal display element is heated to high temperature and particularly, the element is held vertically, for example. In this case, since increase in volume of a liquid crystal material due to thermal expansion becomes greater than increase in capacity of the substrate, extra amount of liquid crystal is needed. As a result, the capacity cannot be absorbed by bending. Therefore, liquid crystal remains in a lower section of the substrate and gap irregularity occurs, so image quality of a liquid crystal display element is deteriorated.
In other words, when a gap holding material with high particle diameter accuracy is independently used so as to make a gap uniform, there arises a problem that spiral gap irregularity which is shown in FIG. 5 occurs according to the above reason at the time of impact. Moreover, since gap uniformity is high, even if slight gap irregularity occurs due to impact, it is visually noticeable, so there also arises a problem that quality of a liquid crystal display element is deteriorated.